AI uses better rally point placement

- AI places rally points at pathable locations. A pathable location isn't strictly required, but avoids the AI setting rally points at seemingly dumb locations. This is an addtional check on top of the existing buildability check.
- AI now evaluates rally points every AssignRallyPointsInterval (default 100 ticks). Invalid rally points aren't that harmful, so no need to check them every tick. Additionally we do a rolling update so rally points for multiple locations are spread across multiple ticks to reduce any potential lag spikes.

OpenRA.Mods.Common/Traits/World/PathFinder.cs

@@ -258,6 +258,25 @@ namespace OpenRA.Mods.Common.Traits
 			return hierarchicalPathFindersBlockedByNoneByLocomotor[locomotor].PathExists(source, target);
 		}
 
+		/// <summary>
+		/// Determines if a path exists between source and target.
+		/// Terrain and a *subset* of immovable actors are taken into account,
+		/// i.e. as if a subset of <see cref="BlockedByActor.Immovable"/> was given.
+		/// This would apply for any actor using the given <see cref="Locomotor"/>.
+		/// </summary>
+		/// <remarks>
+		/// It is allowed for an actor to occupy an inaccessible space and move out of it if another adjacent cell is
+		/// accessible, but it is not allowed to move into an inaccessible target space. Therefore it is vitally
+		/// important to not mix up the source and target locations. A path can exist from an inaccessible source space
+		/// to an accessible target space, but if those parameters as swapped then no path can exist.
+		/// As only a subset of immovable actors are taken into account,
+		/// this method can return false positives, indicating a path might exist where none is possible.
+		/// </remarks>
+		public bool PathMightExistForLocomotorBlockedByImmovable(Locomotor locomotor, CPos source, CPos target)
+		{
+			return hierarchicalPathFindersBlockedByImmovableByLocomotor[locomotor].PathExists(source, target);
+		}
+
 		static Locomotor GetActorLocomotor(Actor self)
 		{
 			// PERF: This PathFinder trait requires the use of Mobile, so we can be sure that is in use.